Planographic offset printing masters



United States Patent 3,547,641 PLANOGRAPHIC OFFSET PRINTING MASTERS Ralph Kingsley Blake, Westfield, N.J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed .Iune 20, 1966, Ser. No. 558,617 Int. Cl. G03c N48 US. Cl. 96--76 12 Claims ABSTRACT OF THE DISCLOSURE This invention relates to photolithographic reproduction; namely, lithographic printing plates. Specifically, this invention relates to photosensitive silver halide elements which can be converted to lithographic printing plates.

Various photographic methods are employed in the lithographic art for the formation of an oleophilic, that is, greasy ink receptive, image on an oleophobic, that is, greasy ink repellent, surface of a lithographic printing plate. In photographic methods, a negative is usually produced in a camera or by contact. Subsequently, the lithographic plate, in which the lithographic surface has been sensitized with a material such as a resin-forming diazo compound or with a dichromate compound capable of tanning a colloid when exposed to light, is exposed to light-through the negative whereby the oleophilic exposed areas are hardened. The light sensitive materials remaining in the unexposed portions of the plate, or the nonimaged areas are removed, e.g., by washing, to provide an oleophobic surface in the non-image areas. Such plates may be mounted on a cylinder of an offset printing press, for example, and passed in contact with an aqueous fountain solution and a printing ink. The inked image is then transferred to a blanket, and applied to the sheet to be printed. This entire operation is very time-consuming in spite of its great number of commercial applications. Consequently, there is a demand for a simple and convenient lithographic printing plate as well as a process for producing such a plate.

The product of this invention in its broadest sense comprises a support bearing an unexposed photosensitive silver halide emulsion layer, said layer having on its outer surface thereof, a transparent, hardened, nonremovable, water permeable, colloid layer containing a nucleating agent suitable for the deposition of a silver image at or on the colloid layer containing the nucleating agent by means of silver transfer development.

The term nucleating agent as used herein relates to those agents which act as development nuclei of sufficient size to catalyze the silver reduction reaction whereby a negative silver image develops in a previously exposed, light sensitive emulsion layer and the undeveloped silver halide grains dissolve, diffuse, and plate out as an inverse or positive image on the nuclei when the product is developed in a developer containing a silver halide solvent.

The term prenucleation as it pertains to the product of this invention refers to the overcoating of the nucleating agent on the light sensitive area of the photographic product prior to exposure.

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A positive" copy as used herein is defined as an inked image printed on the copy sheet corresponding to the original while a negative copy is defined herein as a printed inked background on the copy sheet with the image not being inked.

The preferred embodiment of this photographic element comprises a gelatino silver halide emulsion containing silver chloride, silver bromide, or mixtures of silver chloride and silver bromide coated in the conventional manner on a film support, and an antiabrasion coating of clear hardened gelatin containing silica particles coated over the emulsion layer and being impregnated with a water-miscible organic solvent containing 1% to 25% water and small amounts of a water-soluble sulfide such as sodium sulfide, in concentrations of 0.1 M to 0.0001 M.

The photosensitive, prenucleated element is converted to a lithographic printing element by exposing the prenucleated element and developing by means of silver transfer development in a conventional low silver halide solvent developer to give a negative image. The exposed, partially developed element is then placed in a second developer of high pH, usually 10 to 13+, containing a high concentration of silver halide solvent, usually Na S O in amounts of 5 gm. to 150 gm. per liter of developer. The undeveloped silver halide in unexposed areas dissolves and diffuses as a silver complex to the sulfide nuclei where it reduces to form a positive, mirrorlike surface silver image Whose surface is oleophilic relative to the surface of exposed areas. In this way an oleophilic/oleophobic surface image is made which can be used for lithographic printing in the conventional manner. In some instances, the positive image may be electrically conductive but this is not a necessary requirement of this invention.

Even though the preferred embodiment of this invention uses a dual bath development process, a mono-bath development process may be used whenever a dual bath process is too time-consuming. Specific examples using a monobath development process are disclosed hereinafter.

In a further preferred embodiment of this invention, the developed image may be treated with adjuvants, as disclosed in US. application by Blake, Ser. No. 401,993, filed Oct. 6, 1964, to make the positive, oleophilic image area more oleophilic and the background more oleophobic. The resulting lithographic printing plate may be mounted on a press such as an offset printing press and many high quality copies produced.

The following examples illustrate this invention but are not intended to limit it in any way.

EXAMPLE I The following solution was prepared:

Solution A An aqueous solution of 0.1 M sodium sulfide was diluted with ethanol to give a final concentration of 1 part of 0.1 M aqueous sodium sulfide and 19 parts of ethanol.

A clear-backed orthochromatically sensitized film was prepared by coating an orthochromatically sensitized emulsion of the lithographic type containing a silver salt of 20 mol percent silver bromide, mol percent silver chloride and grams of gelatin per mol of silver halide, on a 0.004 inch polyethylene terephthalate support such as that disclosed in Example IV of US. Pat. 2,779,684 by Alles. An antiabrasion layer of clear gelatin containing 3-12 micron size silica particles and a dimethylolurea hardening agent was coated over the emulsion. Solution A was coated at 70 F. at the rate of 8 ft./min. over the antiabrasion layer. The sulfide nucleating agent of Solution A formed silver transfer nuclei in and on the antiabrasion layer of the lithographic film. The coated prenucleated film was allowed to dry and then exposed through a diapositive having completely clear and completely opaque areas and whose emulsion side was in contact with the base side of the photosensitive element. Exposure was with a General Electric No. 2 Photofiood lamp operating at volts and a distance of 2 feet for 10 seconds. Developer solutions were prepared as follows:

Solution B Water ml 800 Sodium sulfite (anhydrous) gm 80 Hydrogen gm 16 1-phenyl-4-methyl-3-pyrazolidone gm 1 Boric acid gm. 5.5 6-nitrobenzimidazole-nitrate (1 gram dissolved in sufficient ethanol to make up 100 ml. of solution) ml 40 Sodium hydroxide (24 gm. dissolved in sufiicient water to make 100 ml. of solution) ml 100 Benzotriazole 1 gram dissolved in ethanol to make 10 ml. of solution) ml 10 Water to make up to ml 1000 Solution C Developer Solution C was prepared the same as Solution B with the exception that prior to dilution with water to make up to 1000 ml. of solution, 100 ml. of potassium thiocyanate (1 gm./ml.) was added to the developer.

The exposed prenucleated film was developed to a conventional black negative silver image in Solution B at 68 F. for one minute and then drained for five seconds. The film was then developed in Solution C for one minute at 68 F. The remaining unexposed, undeveloped silver halide dissolved and difiused as a silver complex to the sulfide nuclei where it reduced to form a positive mirrorlike silver image on the surface of the film. The developed film was washed in 69 F. water for a period of five minutes and then air dried in five minutes. The resulting positive image was oleophilic relative to the negative image.

The processed film was mounted on an offset press and the emulsion side of the film was rubbed with a 0.01 M lauryl mercaptan solution (1 g. per 100 ml. of ethanol). The film was then rubbed with a 0.3% aqueous colloidal silica solution prepared by diluting with water, a colloidal silica solution containing colloidal silica as SiO having a weight ratio of SiO /Na O of 285.1 and an SiO particle diameter of 15 Ill 1.. The printing plate was then retreated with the lauryl mercaptan solution. These treatments increased the oleophilicity of the positive image and improved the difference in the oleophilic/oleophobic area such that printing ink adhered to the oleophilic areas a while no ink adhered to the oleophobic areas. With water as the fountain solution the offset press was set in operation and 500 positive copies were obtained without any change in copy quality.

EXAMPLE II Example I was repeated except that exposure of the film was through the base in a graphic arts camera with an ultraviolet are light source. The image copied was a resolution test chart and a reflection stepped density wedge. The exposure time was only one-fourth that normally used in exposing through the base. The exposed film was processed for one minute in Solution B at 68 F., drained for five seconds, processed for one minute in Solution C at 68 F., washed in water for three minutes and dried with a fan for five minutes. The processed film was treated as in Example I with the lauryl mercaptan and silica solutions. The treated film served as a lithographic printing plate producing a number of good positive copies.

EXAMPLE III The following nucleation solution was prepared:

Solution D Component 1: M1. Water 420 1.5% gelatin solution 6 0.1 M Na S 3 0.1 M NaOH 3 Component 2:

Ethanol 100 0.1 M HgNO 3 Component 3:

Eethanol 100 0.1 M AgNO 4 Component 4:

0.1 M potassium bromide 20 37% formaldehyde-water 45 The film was prepared as in Example I except no antiabrasion layer was coated over the emulsion prior to prenucleation. The components of Solution D were all at room temperature prior to mixing. Component 2 was added to Component 1 in a 30-second time interval and then stirred for 30 seconds. Component 3 was then added to Component 1 in another 30 seconds and the combination stirred for 30 seconds. Component 4 was then added to the mixture in 5 seconds.

The resulting mixture was skim coated over the same type of emulsion as in Example I at room temperature at a rate of 8 f.p.m. The coating was allowed to dry to produce a photosensitive element, overcoated with receptive nuclei. The photosensitive element was then exposed to a No. 2 General Electric Photofiood lamp operating at 20 volts for five seconds at a distance of 24 inches as in Example I.

Developer Solution E was prepared as follows:

Soultion E Solution E was prepared in the same manner as Solution B except that prior to dilution to 1000 ml. with water, 2 grams of potassium bromide and 10 grams of sodium thiosulfate were added to the developer.

The exposed film was then immersed in developer Solution E for 3 minutes, washed with water and dried. The exposed areas developed a 'black, silver image of low electrical conductivity while the unexposed areas developed white, silver images of high electrical conductivity. Both image surfaces showed oleophobic characteristics but when the processed sample was treated with a lauryl mercaptan solution (1 gm. lauryl mercaptan/100 ml. ethanol), for 5 seconds, water washed for 30 seconds and dried, the white silver image corresponding to the unexposed areas became extremely oleophilic while the black chemically developed exposed areas remained oleophobic. This resulted in a lithographic printing plate which was used to produce multiple positive copies as in Example I.

EXAMPLE IV The following was prepared:

Solution F Water ml. 800 Sodium sulfite (anhydrous) gm Hydroquinone gm 16 Boric acid gm 5.5 Potassium bromide gm 2 Sodium hydroxide (24 gm. dissolved in sufficient water to give ml. of solution) ml 100 Sodium thiosulfate (anhydrous) gm 25 Water to make up to ml 1000 The photosensitive emulsion was prepared and coated on a base support as in Example I. No antiabrasion layer was added. Solution D was prepared as in Example III.

Solution D after preparation and mixing, was diluted with an equal volume of ethanol. The ethanol diluted Solution D was overcoated on the emulsion as in Example III, exhibiting improved coating characteristics. After drying of the overcoat, the photosensitive element was exposed through a transparency having printed characters thereon. Exposure was the same as in Example III. The exposed film was then developed for 2 minutes at 68 F. in Solution F and then washed for one minute in water. The developed film was then immersed in a lauryl mercaptan solution C1 gm. of lauryl mercaptan dissolved in 100 ml. of ethanol) for 30 seconds and then dried. The dried processed film had the desired oleophilic/oleophobic characteristics of a lithographic printing plate. The plate was mounted on an offset press and with water as the fountain solution, the press was set in operation producing a number of usable positive copies.

EXAMPLE V The following was prepared:

Solution G Developer Solution G was prepared the same as developer Solution B except that prior to dilution to 1000 ml. with water, ml. of a solution containing 1 gm. of 1phenyl-S-mercaptotetrazole per 100 ml. of ethanol was added to Solution B.

Solution H Solution H was prepared by mixing 10 ml. of Solution B, 190 ml. of Solution G and ml. of a solution prepared by dissolving 128 gm. of Na S O in one liter of water.

The photosensitive film was the same as that used in Example I. The nucleating agent was prepared by diluting an aqueous 0.1 M Na s solution with sufficient ethanol to give a resulting 0.001 M Na s solution. To prenucleate the unexposed film, the film was immersed in the 0.001 M Na S solution for 10 seconds, drained, and dried in 110 F. warm air.

The prenucleated sample was exposed through a positive transparency, to a No. 2 General Electric Photoflood lamp operating at 20 volts at a distance of 24 inches for 10 seconds. The exposure was through the base of the prenucleated photosensitive element. The emulsion side of the positive transparency was in contact with the base of the prenucleated photosensitive element this simulating a camera exposure.

The exposed sample was developed in Solution H at 68 F. for 2 minutes, followed by a 68 F. water wash for 3 minutes. The processed film was placed on an offset printing machine and the emulsion side treated with lauryl mercaptan and silica solution as in Example I. The positive silver image was highly oleophilic and the surrounding area was oleophobic. Using water as the fountain solution, the olfset printing press was set in operation and approximately 100 good quality positive copies were produced.

As pointed out above, the receptive nuclei are overcoated on the emulsion of the photosensitive film prior to exposure and development. The prenucleated layer must be a hardened, nonremovable, colloid layer which is permeable to water. If exposure is through the prenucleated layer, the layer must be transparent. Also, the prenucleated layer may serve as an antiabrasion layer or it may be coated over a separate antiabrasion layer. The antiabrasion layer may contain silica particles but the presence of either the silica or the antiabrasion layer is not required for satisfactory results. If the light-sensitive element does not have a separate antiabrasion layer, the silver surface image will be formed by the nucleating agent at or in the surface of the emulsion, but if an antiabrasion layer is present, the nucleating agent may be incorporated in that layer and the silver image will form in the antiabrasion layer.

During exposure, a negative image is formed in the light-sensitive layer of the photosensitive element. Developing the exposed prenucleated element in a silver transfer developer containing a silver halide solvent causes the silver ions from the unexposed silver halide to migrate to the surface of the light-sensitive layer. The nucleating agents already present at the surface serve as the starting point for reduction of the silver ion complexes to metallic silver. Suitable agents which serve as the nuclei to begin the reduction reaction are finely dispersed heavy metals, metal sulfides, selenides or tellurides and water soluble salts which are in equilibrium with sulfides or selenide ion or salts which decompose at development to give elemental mercury and/or mercury sulfide. Examples of these nucleating agents are colloidal mercury, silver, gold and their sulfides, sodium sulfide, sodium selenide, sodium thiocyanate, thiourea, mercurous chloride, and mercurous nitrate.

The development process may consist of a dual bath or a single bath process. If a dual bath process is used, the first developer is a conventional low silver halide solvent developer. These low silver halide solvent developers usually contain hydroquinone with small amounts of a silver halide solvent. The second bath is a high pH developer of the super additive type containing relatively large amounts of silver halide solvent, e.g., 1-phenyl-4- methyl 3 pyrazolidone/hydroqui.none developers containing sodium thiosulfate. Suitable silver halide solvents include sodium, ammonium, and potassium salts of thiocyanate, thiosulfate, and sulfites. Monobath developers containing high concentrations of silver halide solvents may also be used whenever development time is a factor and the dual bath system cannot be used.

To obtain good prints, it is necessary that the oleophilic image accept sufficient ink for lithographic printing while the oleophobie background of the image accepts no ink at all. To promote the distinction between the oleophilic/oleophobic properties, the surface containing the positive silver surface image may be treated with an organic adjuvant containing an oleophilic group and an argentophilic group usually selected from the followmg:

(1) SH or a group enolized to SH.

(2) SR where R is an easily hydrolyzable group to give SH, such as an acyl, amidinium or NHR where R is an alkyl, aryl, aralkyl, cycloalkyl, or heterocyclic group.

(3) A thioacid group.

(4.) A thioamide group.

(5) A selenium analogue of groups (1) through (4).

To render the oleophobic areas more oleophobic, the printing plate may be treated with a low concentration colloidal silica solution containing SiO- and having a particle diameter of 15 me.

The film to be exposed preferably should be transparent so that it can be exposed from the front or back of the film, that is, either direction. But the film does not have to be transparent because a film with an opaque backing, etc., can be exposed from the front and subsequently developed so that there: is a necessary silver transfer to the unexposed surface region.

The preferred method of exposure is in a camera through the base or to the prenucleated emulsion using an image reversing prism so that there is lateral image reversal. In that manner, a correctly oriented positive copy can be obtained when the light-sensitive element is used as a printing plate.

The organic colloid of the silver halide emulsion layer used in this invention is not especially critical and may be gelatin or other natural or synthetic organic colloid binding agents. Suitable colloids includes those disclosed in US. Patents 2,276,322; 2,276,323; 2,347,811; and 2,833,050.

Similarly, the silver halide emulsion may be selected from well known emulsions containing silver chloride,

silver bromide and mixtures thereof, as well as mixtures containing small amounts of silver iodide. The emulsions may also contain optical and chemical sensitizing agents, fog-stabilizing compounds, emulsion hardeners, plasticizing compounds, wetting agents, toners, and matting agents.

The film support for the emulsion layers used in this novel product may be any suitable transparent plastic. For example, the cellulosic supports, e.g., cellulose acetate, cellulose triacetate, cellulose mixed esters, etc. may be used. Polymerized vinyl compounds, e.g., copolymerized vtinyl acetate and vinyl chloride, polystyrene, and polymerized acrylates may also be mentioned. The film formed from the polyesterification product of a dicarboxylic acid and a dihydric alcohol made according to the teachings of Alles, U.S. Patent. 2,779,684, and the patents referred to in the specification of that patent. Other suitable supports are the polyethylene terephthalate/isophthalates of British Pat. 766,290 and Canadian Pat. 562,672 and those obtainable by condensing terephthalic acid and dimethyl terephthalate with proplyene glycol, diethylene glycol, tetramethylene glycol or cyclohexane 1,4-dimethanol (heXahydro-p-xylene alcohol). The films of Bauer et al., U.S. Pat. 3,052,543, may also be used. The above polyester films are particularly suitable because of their dimensional stability.

As the support for the planographic printing form, it is possible to use plates consisting of metals, alloys, or metal oxides coated on metal.

Besides immersing the film in the nucleating agent other methods of applying the solutions can be used such as dip roll, hopper, spray and the like so that sufficient amount of solution is applied to the surface of the photosensitive element.

When making conventional printing plates an ordinary negative is made and this is used for making a positive oleophilic image on the printing plate by contact exposure. The step of producing a negative is eliminated by this invention since the making of the negative is replaced by the direct preparation of the printing plate described above.

Further advantages of this invention are evidenced by the fact that the invention provides a new and rapid method of making planographic printing plates by starting with a photosensitive material having any desired spectral and sensitometric characteristics within the range of present emulsion technology.

A further advantage is the ability to make planographic masters from camera speed film. This invention offers a further benefit, the presence of the receptive fogging nuclei coated over the emulsion prior to exposure eliminating the extreme sensitiveness in the concentrations of the nucleating agent and the silver halide solvent when both are incorporated in the developer.

The product of this invention presents a versatile and dependable planographic printing plate which has oleophilic/oleophobic characteristics needed in the lithographic printing industry.

The embodiments of the invention in which an exclusvie property or privilege is claimed are defined as follows:

1. A photographic element for the production of lithographic plates which consists of a support bearing a photosensitive silver halide layer and said layer having an outer surface layer of a hardened, nonremovable, water-permeable organic colloid containing a silver nucleating agent.

2. An element as defined in claim 1 where said colloid layer containing the nucleating agent is an anti-abrasion layer.

3. An element as defined in claim 2 where said nucleating agent containing layer comprises a hardened gelatin colloid having silica particles dispersed therein.

4. An element as defined in claim 1 where said support is a polyester.

5. An element as defined in claim 4 where said polyester is oriented polyethylene terephthalate.

6. An element as defined in claim 1 where said silver halide is silver chloride, silver bromide or mixtures of both.

7. An element as defined in claim 1 where said nucleating agent is a water soluble metal sulfide.

8. An element as defined in claim 7 where said nucleating agent is sodium sulfide.

9. An element as defined in claim 1 where said nucleating agent is mercury, mercuric sulfide, or combinations thereof.

10. A process for making lithographic images which comprises:

(a) developing a photographic element which consists of a support bearing a water-permeable colloid silver halide emulsion layer, said layer having an integral outer surface layer of a hardened, non-removable, water-permeable, organic colloid containing a silver nucleating agent to form a negative image;

(b) forming by diffusion silver transfer development in a silver halide developer solution free from organic compounds containing an oleophilic group, an oleophilic positive silver surface image in said outer surface layer to provide a lithographic printing plate;

(c) treating said outer surface layer with an aqueous solution and a printing ink containing an oleophilic binding agent, whereby said ink adheres to the areas of said surface corresponding to the positive silver surface image; and

(d) using said plate to print by lithography.

11. A process according to claim 10, wherein steps (a) and (b) occur simultaneously.

12. A process according to claim 10, wherein steps (a) and (b) occur stepwise in the order given.

References Cited UNITED STATES PATENTS 2,352,014 6/1944 Ratt 96-29 2,789,054 4/1957 Panel 96-29 2,843,485 7/1958 Yutzy et al. 96-29 2,878,121 3/1959 Wray 96-29 2,944,894 7/1960 Panel 96-29 2,969,014 1/1961 Hanson et al 96-29 3,053,659 9/1962 Panel 96-29 3,161,508 12/1964 Hepher et al 96-33 3,260,600 7/1966 DeHaer 96-29 3,278,958 10/1966 Regan et al 96-29 FOREIGN PATENTS 746,948 3/ 1956 Great Britain 96-29 565,696 10/1958 Belgium 96-29 272,658 10/ 1963 Australia 96-29 OTHER REFERENCES Modern Plastics Encyclopedia, 1965, vol. 42, McGraw- Hill Publication, p. 472.

NORMAN G. TORCHIN, Primary Examiner J. R. HIGHTOWER, Assistant Examiner U.S. C1. XR. .96-33 

